The consumable electrode furnace process has been in use for a number of years for the production of purified and defect free ingots. The present invention is applicable to both the vacuum arc and electroslag process. For simplicity of explanation, the electroslag process will be discussed. Essentially the process comprises positioning the consumable electrode within a crucible above a molten slag pool and passing a high current from the electrode through the molten slag into the crucible base. The electrode is progressively melted and reformed as a purified and defect free ingot in the crucible body.
As discussed in U.S. Pat. No. 3,684,001 of which I am a coinventor, any return bus bar configuration which is not coaxial with the current path formed by the electrode, molten pool and ingot, sets up powerful stray fields in the melting zone. The interaction between the vertical components of these fields and the horizontal components of melting current in the molten pool, stirs the metal with such violence as to cause unacceptable segregation at economically high melt rates. A return bus bar which is truly coaxial eliminates this problem by eliminating all vertical components of magnetic field in the melting zone.
One of the first attempts made to reduce the effect of magnetic stirring of the molten metal is the method and apparatus disclosed in U.S. Pat. No. 3,684,001. In this apparatus, the return path for the current was taken from the base of the crucible upwardly through a plurality of legs running vertically from the base of the crucible alongside but external to the outer jacket of the crucible. In this structure, the current flow in the legs was in a direction opposite to the current in the path formed by the electrode, slag, molten metal, ingot and base of the crucible.
The resultant countercurrent flow in the crucible and that in the legs provides opposing magnetic fields which tend to cancel the stirring effect. However, the use of legs requires additional structure and expense in the construction of the furnace. Additionally, the external legs do not present a full coaxial situation, but, instead, some field distortion still exists and magnetic stirring is not completely eliminated.
Another structure which is touched upon lightly in U.S. Pat. No. 3,684,001 and which is in public use is to use the innercrucible body itself as the return conductor. In this structure, the upper flange of the crucible body is electrically interconnected to the return current path. Accordingly, the current flow pattern in this structure is downwardly through the electrode into the molten slag, metal and ingot, into the base and thence upwardly through the crucible wall to the return flange.
Using the crucible body as the return current path also has certain drawbacks. One of those drawbacks is that arcing can occur between the solid ingot and the inner wall of the crucible which tends to destroy the crucible itself.
Another requirement in a crucible for the consumable electrode melting process is removal of substantial quantities of heat through the relatively small area of crucible wall which is in contact with the molten slag and metal pool at any given time during the process. For example, a 5 ton electroslag ingot of 20" diameter using 15,000 amperes of melting current at 30 volts drop across the melting zone is transferring almost 450 kw of heat through a six inch high zone of water-cooled copper crucible wall. This heat transfer rate may be expressed as: ##EQU1##
To support this high rate of heat flow, the outer wall of the copper crucible tends to rise to a temperature above the boiling point of water, thereby creating steam at the copper-water interface. Since steam is an excellent thermal insulator, effective cooling ceases, the copper temperature rises into the 600 to 900 degrees F. range and the crucible body becomes dead soft annealed so that minor mechanical abuse during stripping, handling or cleaning leads eventually to major repairs or scrapping of the crucible.
Experimentation has shown that the best way to remove the steam film as rapidly as it forms is by applying ultrahigh velocity cooling water to the outer surface of the copper crucible wall. A cooling water velocity of at least 10 feet per second has proved to be required, and this velocity must be at the surface of the copper, not merely at the center of a substantial cooling passage of which the crucible copper is one of the walls.
At the preferred cooling water velocity of 20 feet per second, assuming a quarter inch wide cooling water annulus, the volumetric flow is: ##EQU2##
Such velocities require high flow rates through small passages, thereby generating pressure drops of the order of 20 to 60 psi, depending on surfaces, shape and length of crucible.
I have conceived that by wrapping a thin copper sheet around the crucible body, spaced away from the crucible by insulators parallel to the direction of flow, a combination ultrahigh velocity cooling water guide and truly coaxial return conductor may be formed.
Since all magnetic field is internal to the coaxial conductor, the outer crucible water jacket structurally strong enough to support the weight of the ingot and the copper crucible, may be fabricated inexpensively from carbon steel.